The contribution of diesel exhaust (DE) to health, especially children's health, is of tremendous public health interest. DE has been associated with worsening asthma and allergies, among other important health effects. Reducing DE exposures has become a major regulatory initiative, and federal, state, and local jurisdictions are investing hundreds of millions of dollars in retrofitting diesel engines in school buses and other changes to reach this goal. However, a serious question remains unanswered: what magnitude of impact will this major governmental environmental policy have on public health, especially the health of asthmatic children who commute by diesel bus? This proposed study fills in the knowledge gap and provides substantial public health information on the impact of such regulations and nationwide changes by examining the importance of DE exposures to asthma morbidity among children who ride school buses, while adjusting for the influence of ambient air pollution and other factors that change over time. This 5-year study will be the first to assess children's health effects from DE before and after the change from "dirty" to "clean" engines and fuels in school bus fleets. This project aims to: 1) determine whether asthmatic children changing to buses with cleaner fuels and engines have a reduction in sub-clinical and clinical asthma severity; 2) determine if increased levels of DE exposure lead to an increase in clinical features of asthma; and 3) quantify and characterize the changes in particle and toxic gas exposures to DE in asthmatic children. This study includes 3 years of exposure and health monitoring on 500 asthmatic school children who commute to school in the Puget Sound area of Washington State. Monthly measures of pulmonary function using spirometry, pulmonary inflammation assessed by exhaled nitric oxide, asthma symptoms, medications, and clinical encounters, will be obtained on all children during the school year. The 500 children will be recruited from several school districts to represent 3 exposure scenarios: 1) 125 children riding retrofitted diesel bus or commuting by car over all years of study; 2) 250 children riding old diesel buses, which will be retrofitted after the first and second year of the study; and 3) 125 children riding old diesel buses throughout the study. Exposures will be estimated for all subjects, and an in-depth individual exposure assessment will be performed on a subset of 144 subjects and 48 school buses in each monitoring year to cover the spectrum of bus type and routes. These personal exposure measurements will be used to characterize and distinguish children's exposure to DE from other sources, and develop an exposure model for those who are not personally monitored. This study will provide critical missing information relating reductions in diesel emissions directly to reductions in exposures and health effects.